**From New Scientist #2751, 13th March 2010** [link]

Joe has a special set of white counters numbered on one face 1, 2 or 3 and some of them have the other face coloured green. To test Penny’s logic skills, he placed 36 of his special counters on the table to form a 6 by 6 grid (see right). The number on the top face of a counter indicates the number of counters touching it that have a green lower face. Penny had to find which had green faces.

Numbering the rows and columns from the top-left corner, in which columns are the green-faced counters in the third row?

[enigma1586]

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This Python program runs in 37ms:

Solution:The green faced counters in row 3 are in columns 2, 3 and 4.